Composite set



Julyz'l, 1937.

H. WHITTLE COMPOSITE SET Filed March `1-1 1936 IV 5.153.# S Al bww@ Patented July 27, 1937 ENT oFFlC'I-ifff COMPOSITE SET 'i Horace Whittle, Maplewood, N. J., assignorl to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation'of New York Application March 11, 1936, Serial No. 68,261

This invention relates to multiplex signaling systems and more particularly to improvements in arrangements such as composite sets usually associated with the terminal facilities of the lines of 'such systems to permit the different currents utilized from interfering with each other.

In a signaling system in which both telephone and telegraph currents are transmitted over the line circuitsthere'of, it has beenthe practice to associate, -with the terminal facilities of such line circuits, a device known as a composite set whereby the incoming telegraph and telephone currents may be separated and diverted to their respective circuits without interference with each other. -Thecomposite set furthermore functions to prevent outgoing telegraph currents from entering'the telephone parts of the terminal facilities and vice versa.`

It is the primary object of this invention to improve composite sets by providing novel means for preventing `the telegraph current originating at the terminal of the line from entering the telephone circuit at the same terminal and thereby obviate any disturbances which might be caused in thetelephone circuit bythe telegraph signals cominginto the termina Inaccordance vwith the present invention the composite set makes use of a wave lter of ther constant resistance type whose elements arecomputed by the method described inl Patent 2,043,345, issued to S. Bobis, on June 9, 1936.

A clearer conception'of the scope'and purpose of the invention may be obtaine'd from the following description and attached drawing, which L shows a circuit arrangement forobtaining a Morse`composite set exhibiting a substantially constant :resistanceover the greater -part of the operating frequency range." 'I'he circuit elements of the set `are connected to an vordinary telephone interoice trunk and consist of retardation coils A,.Gr', D and K, condensers B1, B2, C1, C2, E1 and E2, resistancesF1 and F2, and repeating coil H1,

arranged in vrelation to each other as shownvand located at loneend of the interoiice trunk with r similar elements similarly arranged at'theother end of said trunk and both ends arranged fory operation with a compositeinetallic telegraph circuit or f two composite grounded telegraph circuits. t

The path of the telegraph signal is as follows:

The'signal from anytrans'mitting device capabe of sending out telegraphic impulses reaches the com# posite set; say at X1, flows through the upper-coils A and thence through the line Wire Y, through V uppercoils KY'` and A at the far end of the line,

and thence to a telegraph receiving instrument at X2. With'a grounded telegraph system the re-g` turn .current is, of course, transmitted throughV the ground while with the metallic system in whichthetwo conductors X2 and X3 would be,7 joined at the telegraph sending and Vreceivingapparatus at the east end of lthe line and the conductors X1 and'X4 would be joined at the telegraph sending and receiving apparatus at the West end of the line,4 the returnxcurrent flows again into the composite set at X3, through lower coils A' and K', line Z, through lower coils A and' K' backrvto the metallic telegraph sender at X4. j i iv Simultaneously, telephone conversations origi'i hating at telephone Vstation extending, say, from. a telephone line connected to repeating coil H1v on the Westside ofthe line,'enter the composite terminal through the! repeating coil'H1 and are transmittedthrough condensers Bi,-B2 and'C1;l C2 over the line wires Y'vand Z as a two-wire@ metallicy circuit,through condensers B1', Bzrar1'cl C1" C2' at the far end, andrepeating coil H1 to ai telephone station extending from la telephone line connected tothe repeating coil "I-I1. Telegraph currents are prevented from linterfering. with this telephone-conversation by the presence of condensers B, C and E at each end of the trunk, while retardation coils A and K, whichprovide a readylpassage tothe telegraph currents, exclude the telephone currents from the telegraph circuit. Y Coils D and G provide a by-pass to vground for any residual telegraph' currents which mayy getl through condensers EBl and E.

The theory of my substantially constant resistance Morse composite circuit is as follows: When the ii'npedance'hofv the subscribers circuit, as seen through-the--interofce trunks and oce' circuit, issubstantially-a resistance, which is the usual case in:telephone-practice, then Athe impedancemeasured acrossthe line sidelof the re-v peating coils H1 indicated by arrows will be sub stantially a resistance. Ifwe'substitute Aa resist-y ance for 'the two windings of the repeating coil and makethese resistances equal to resistances F1 and F2,-the structure will be a balanced form of the constantresistance low-passlter disclosed inl theabove mentioned patentto'S. Bobis: If we setI the critical or cut-off frequency'. ofs'this low-pass lterat some 'frequency abovethe telegraph and below'the telephone range, say. at 80 cycles, and the image impedance isn such as to match thatl of the line, then the constants ofthis crcuitcan be obtained by .using the'principles laiddown in the above mentioned patent.

There are several features of my arrangement, however, which should be pointed out. Ii the telegraph arrangement is used as two grounded telegraph circuits each using one side of the line as shown in the drawing, then, in order to pre- Vent cross-fire or interference between the telegraph circuits, there should be no mutual inductance between coils A and K. Separate cores for the two windings, however, would introduce unbalances in the telephone circuit inasmuch as the two cores would vary their inductances inde.

pendently when telegraph circuits ywere passing through them, which would cause cross-talk or noise in the telephone circuit. Therefore, for grounded telegraph operation,"coils A and K should be wound on two separate cores with the windings arranged in the same way as in the retard coil disclosed in Patent No. 1,762,744, granted tol E. L. Schwartz et al on June 10, 1930.

The mutual inductance of repeating coil H1 should be suiiiciently high so as not to aiiect the impedance of the subscribers line as viewed through the repeating coil. With the circuit arrangement shown, and with the'` element values determined asabove noted, the circuit operates approximately as a balanced bridge, the impedance at `the line side oi the repeating coil H and the impedance of the telegraph terminals being conjugate arms of the bridge. This means that impulses entering the circuit through the telegraph legs X will cause no voltage to-be irnpressed across the line windings of repeating coils H1 on the same end of the circuit. Thesubscriber on the line connected to repeating coil- H1 on the west side of the line, for instance, will not ybe affected by .the so-called Morse thump when telegraph impulses are sent out over telegraph legs Xi and X4.v Likewise, the subscriber connected to repeating coil H1 on ,the east side of the line will not be aected by the impulses sent out over telegraph legs X2 and X3. 'I'his balance, however, would not .prevent the latter subscriber from receiving impulses from telegraph legs X1 and X4.. Nevertheless, these irnpulses are attenuated materially by theV line YZ and the filtering action of the.` composite sets, which is usually sufficient to ,prevent interference. The benefit of the bridge balance is to reduce noise in the telephone circuit by .practically eliminating near end interference from the telegraph into the telephone circuit. The only condition necessary to maintain this balance' is that the impedance of the line YZ and resistances F1, F2 in series, or resistances F3 and F4 in series, should be equal at telegraphic frequencies and their harmonics.

Another condition of balance` would obtain if the telegraphic terminal impedance and the telephone subscribers line impedance lwere equal. Under the latter condition, signals coming in over vthe line would not produce a difference in potential across resistances F. However, this condition has no advantage from the standpoint of discriminations between telephone and telegraph signals. Nevertheless, if the telephone and telegraph terminal impedance were made equal to the line impedance and also equal tothe sum of the resistances F1 and F2, then this set would exhibit, in addition to its other advantages, a constant resistanceimpedance tothe lineat all frequencies, which :is desirable in preventing echoes `andsinging in repeater circuits.

What is claimed is: j Y l. .A transmission line, a v.connecting vmeans serially included in said line, atelephone circuit associated with said line through said connecting means, a composite set including a telegraph bridge across said line, telegraph legs associated with each side of said line circuit through said bridge, and means within said composite set for making its characteristic impedance substantially a constant pure resistance for the range of operating frequencies when .said impedance is measured from the transmission line side.

2. A constant resistance composite network 'for separating telephone and telegraph currents comprising a, telephone line circuit, grounded telegraph legs, a retardation coil system having a set of two cores with a plurality of coils disposed thereon, another set of two cores with a plurality of coils disposed thereon and having a common flux linkage and portions of their windings so poled so as to practically eliminate mutual couplings between the coils, condensers connected between the windings of said two sets of cores on the one side, condensers connected between the windings of said two sets of cores on .the other side, resistances connected `to A,one side of', said rst-mentioned condensers -extending to:

said telephone line circuit and said grounded telegraph legs connected to the other side of said first-mentioned condensers.

3. A constant resistance composite network for.

separating telephone and telegraph currents comprising a telephone line circuit, ametallic4 telegraph circuit, a retardation coil system have ing a common ux linkage comprising three coresy with a plurality of coils disposed. thereon, con.- densers intermediate the windings of a second core and a third core, condensers intermediate: the windings of said first core and saidg-third.

core, resistances connected to one side of said first-mentioned condensers extending to saidte'le` phone line circuit and said telegraph metallic circuit connected to the other side-ofsaid rstmen' tioned condensers.

4. A constant resistance composite'network forV separating telephone and telegraph currents hav-- ing one set of opposite arms comprising a line' circuitincluding a capacity on each side thereof` and a balancing network, and another set ofr op-z posite arms comprising an inductance and a rcapacity respectively, each ofA the latter joined to each of the balancing networks, a `telegraph leg connected to the junctionpoint of each of saidinductance and capacity, a telephone circuit con` nected to the junction point of said capacity'in series with eachof the arms of said vline circuit,y

a retardation coil system bridged across the ca-4 pacity side of said yline circuit and grounded at its midpoint, and another retardation coil system bridged across the vbalancing network and grounded at its midpoint.

5. A line circuit, a constant resistance bal-- ancing network comprising a resistance, a capacity and inductance all in series on one side of the line, a similar resistance, capacity and an inductance all in series on the other side of- 'the' line, a retardation coil system bridged across side line circuit and another retardation coil system bridged across said condensers, all elementsof said network being so arranged to form a bridge substantially balanced for all transmitting frequencies, and a telephone circuit and a telegraph circuit so associated with said elements as ito be in conjugate relationship with eachother and with said line circuit. Y

6. Av constant resistance bridge substantially balanced for al1 transmitting frequencies 'having one set .0f .opposite arms comprising alinecircuit and a balancing network of a resistance element and a capacity element, and another set of opposite arms, comprising an inductance and a capacity, signaling circuits of different types connected to the opposite junction points of the arms of said bridge, a balanced retardation coil system connected between the resistance elements of said balancing networks and said inductances, and another balanced retardation coil system connected between the capacity elements and resistance elements of said balancing network.

7. A transmission line and aI composite set associated therewith, said composite set comprising a telephone circuit and a telegraph circuit so arranged as to be in conjugate relationship to each other and having the constants of said circuits so adjusted that the impedance presented to said circuits from said transmission line side will be substantially equal to a. common pure resistance.

HORACE WHI'I'ILE. 

